The present invention is directed to an improved method for manufacturing a data disc having embossed servo information to provide a smoother surface.
Data storage on rotating media requires position sensing information to be included on a part of the data storage surface so that the data storage systems can retrieve information recorded on that surface. Prior art devices have traditionally used different methods for encoding and storing this position sensing information depending on the type of media and reading mechanism used.
Traditional Winchester magnetic storage systems have used magnetic signals recorded in the thin-film media surface for this purpose. These signals are typically recorded by the same systems used to write data to that surface in a process known as servo writing. The servo writing records information on the media about the identity and location of the data which is then used during the writing and reading processes to derive servo signals which allow for the accurate positioning of the head during these operations. Typically the data is arranged in a concentric series of tracks, each track being made up of a number of sectors, which in turn contain multiple bits of binary data. Since these signals are individually recorded by a single writing head on each recording surface of the storage system, the time required to complete this process is becoming increasingly lengthy as the total number of sectors and tracks increase.
Magneto-optical systems have traditionally used embossing processes which were originally developed for use in read-only, compact disc storage systems. Here the servo sector information is written using optical lithographic systems on the surface of a master disc. A series of pits and grooves is formed in this surface, which is then replicated onto the surface of a metal mold. Numerous plastic discs with accurate copies of this pattern are produced by injection molding processes. Since the molding operation is fast and low cost, the complete servo information is provided on the entire disc surface with this inexpensive process, making writing of individual sector information on the disc unnecessary.
Hybrid data storage systems have been previously described whereby a flying optical head is used to write and read data by means of a magneto-optical system. The media used in this system is similar to prior art magneto-optical system in that embossed servo information is used to locate the position of the head as it is flying over the media. The media is of the so-called first-surface variety, in that the thin film magnetic layer is on the outside surface of the data storage disc, rather than underneath a rather thick protective layer as is commonly used for prior art magneto-optical data storage systems. The embossed servo pits and grooves in a first-surface media are directly underneath the air bearing surface of the flying optical head, which affect the ability of the head to fly uniformly over the recording surface.
In such system the embossed pits are required to have very tight dimensional tolerances to perform adequately in their intended use. The depth of the pits and grooves must be controlled to a specific fraction of the wavelength of light used, for example, xc2xc wavelength of 650 nm light. The observed servo signal is due to destructive interference between light reflected off of both surfaces, so that changes in the pit depth result in changes in the magnitude of the reflected optical signal. Since interference is used to generate the signal, significant lateral changes in the size of the spot can cause the adjacent pit edges to effectively overlap, reducing the magnitude and distorting the shape of the servo signal.
In traditional Winchester magnetic recording systems, the maximum data storage density is set by the product of the linear recording density, that is, the number of bits that can be recorded along the path of the flying head, and the track pitch, that is, the number of tracks per radial dimension on the rotating disc. The linear recording density is primarily set by the gap between the media and the writing and reading heads, assuming that the film thicknesses of the heads and discs can be made thin enough as described by the so-called Wallace and Potter equations. The track pitch is set more by the ability to position the head over a particular track, the fabrication control over the pole gap width, and the ability to minimize distortion of adjacent track information when writing a particular track. Limitations in areal data storage density in prior art systems are primarily due to these track pitch limitations.
This invention is directed at processes that use traditional embossing techniques to provide the servo and track location information required by both magnetic and magneto-optical systems. These processes then modify the embossed grooves by filing them with various materials and polishing the surface so that the embossed pits and grooves are selectively filled with the deposited material. In this way, the surface is made flat enough to provide a smooth surface for flying a head very close to this surface while maintaining the economical advantages of molding the servo information into the disc surface. The material in the filled grooves can be used for sector identification and track following and also as a magnetic or thermal barrier between adjacent tracks.
A primary element of this invention is the use of a differential removal process such as chemical-mechanical polishing (CMP), which is a process primarily used in the integrated circuit industry to control planarity of deposited and patterned layers. The deposited layers used to form insulating and conductive regions in integrated circuits are generally conformal, in the sense that their as-deposited thickness is constant regardless of the topology of the underlying regions. As multiple layers are deposited, patterned and etched, it becomes increasingly difficult to correctly perform the lithographic steps on surfaces that are no longer smooth and flat. Thus polishing steps are incorporated after deposition steps to return the surface to being flat and smooth, after which the required lithographic steps can be performed with sufficient accuracy. Both equipment and processes have been developed to polish various layers in the presence of other layers such that there is a large selectivity on the removal rate between different layers. The layers with the lower polishing rate form so-called etch-stops (actually polish-stops), which prevent further polishing after the lower polishing rate material is exposed.
The invention includes the formation of a master pattern of servo and track information and the subsequent transfer of that pattern to a series of pits and grooves on a substrate. On top of that substrate, at least one sacrificial layer is provided atop a relatively hard layer. By sacrificial layer it is meant that the layer is relatively easy to etch or otherwise remove in a controlled, planar step. By a hard layer, it is meant that the layer is relatively polish or etch resistant. A data storage layer may serve as this hard layer.
For example, in a magneto-optical design, the recording stack may be provided with both silicon nitride and silicon dioxide top layers, with the silicon dioxide layer acting as a sacrificial layer to ensure that the hard layer, of silicon nitride, remains at the end of the process. In a further alternative, a layer of aluminum or aluminum alloy may be deposited, with the aluminum plugs filling the grooves and pits (created by the embossed servo information) to a level higher than any of the adjacent layers of silicon dioxide, silicon nitride, or similar dielectric layer. Since the polishing rate of aluminum can be far faster than that of the silicon dioxide, then the aluminum can be etched or otherwise removed down to a level equal to or slightly below a planar surface with the silicon dioxide, with the silicon dioxide layer allowing for some small level of over polishing. The net result would be that the silicon nitride layer would be protected completely; the silicon dioxide layer would partially remain and partially be removed; and the aluminum metal which fills the grooves and pits would rise only to a level substantially equal the very flat top surface of the silicon dioxide. Of course, alternative filler materials could be used in a similar process as long as an appropriate selective removal process is available with sufficient selectivity. In this example, the aluminum functions as a sacrificial layer; the silicon dioxide is effectively serving as a xe2x80x9chardxe2x80x9d layer, as it is removed more slowly. In an alternative, the silicon dioxide layer could be omitted, with the silicon nitride layer now being the xe2x80x9chardxe2x80x9d layer.
For conventional Winchester magnetic recording discs, the grooves could be filled with a non-magnetic material such as aluminum, glass or polymer, such as polyamide, or a magnetic material of higher or lower permeability, coercivity, or susceptibility and polished smooth. Such filler material again is selected on the basis of its removal selectivity relative to the basic xe2x80x9chardxe2x80x9d material of a magnetic recording disc.
Other features and advantages of the present invention will become apparent to a person of skill in the art who studies the following invention disclosure given with respect to the following disclosure.